1. Field of the Invention
The present invention relates to a method of manufacturing an optical glass element, and an optical glass element manufactured using the method, and in particular to a method of manufacturing a prism, and a prism manufactured using the method.
2. Prior Art
An example of an optical glass element used for deflecting light rays in precision optical instruments is a small prism. A prism is a transparent body having two or more optical surfaces, wherein at least one pair of surfaces are not parallel or even approximately parallel to one another, and is made of an optically isotropic material such as glass. Such prisms are classified into erect prisms that deflect light by reflection, and spectral prisms such as pentagonal prisms that split light into monochromatic light by means of optical dispersion. A basic spectral prism is a triangular prism in shape.
With an erect prism or a spectral prism, it is necessary for the flatness, which represents the extent of distortion or deformation of the prism surfaces (this flatness is the maximum value of the offset from an imaginary flat surface), to be no more than the wavelength λ of the light reflected or split by the erect prism or spectral prism. To obtain a sufficient flatness, as shown in the flowchart in FIG. 6, conventional prism manufacturing methods involve repeatedly grinding the prism surfaces and examining the flatness of the ground surfaces.
In FIG. 6, first the glass is melted (step S60), a mother glass is cast from the molten glass (step 61), and the cast mother glass is cut into an approximate prism shape (step S62). Next, the prism surfaces of the mother glass that has been cut into an approximate prism shape are ground to a roughness of #100, then #400, then #600, then #800, and finally #1000, with the flatness being examined and corrective grinding being carried out after each of these grindings (steps S63 to S67). After this, polishing (finishing) is carried out (step 68), then optical coatings are applied to predetermined prism surfaces to make these surfaces anti-reflective, reflective or semi-transmitting (step S69), and then the prism is cut to a predetermined length (step S70), thus completing the manufacturing of the prism.
Japanese Laid-open Patent Publication (Kokai) No. 10-1321, on the other hand, discloses a method of manufacturing a prism in which a rod-shaped glass element is prepared from a mother glass using a hot drawing method, and then a long prism is obtained by press forming the glass element while softening by heating.
However, there are problems with the method of manufacturing a prism shown in FIG. 6, in that a large mother glass is gradually reduced in size by repeatedly grinding and examining a number of times until a prism of predetermined dimensions is obtained, and hence manufacturing the prism is time-consuming, and moreover, if the prism has a polygonal cross section and thus a large number of surfaces, then the number of surfaces to be ground increases correspondingly, and hence an excessive amount of time is required. Furthermore, depending on the cross-sectional shape, a special jig may be required, resulting in increased machining costs, and moreover corners may be chipped or the like during the grinding, resulting in a reduced product yield.
Moreover, there are problems with the method of manufacturing a prism disclosed in Japanese Laid-open Patent Publication (Kokai) No. 10-1321, in that it is the glass element and not the final product prism that is manufactured by the hot drawing method, and hence even if there is an increase in the smoothness of the prism surfaces of the glass element, the smoothness of the prism surfaces will drop when the prism is press formed.